Method of aluminizing ferrous materials



Dec. 6, 1960 c. J. OWEN METHOD OF ALUMINIZING FERROUS MATERIALS- Filed Feb. 24, 1959 3 zwacou $20 6 5% 5;

25 62 0h QZEEHEEE IN V EN TOR Charles J. Owe/2 BY w United States Patent "ice 2,963,384 METHOD OF ALUMINIZING FERROUS MATERIALS Charles J. Owen, Pittsburgh, Pa., assignor to Hubbard and Company, Chicago, 111., a corporation of Pennsylvama Filed Feb. 24, 1959, Ser. No. 794,812 8 Claims. (Cl. 117-52) This invention relates to methods of coating ferrous metal articles with aluminum or aluminum base alloys.

Aluminum coated steel is a superior product for use in a marine or industrial environment because it exhibits a good resistance to salt and acid attack and a good resistance to heat. However, appearance-wise, aluminum coatings are generally rough and dull thereby providing a surface which is not only unattractive but also a surface which can be attacked by the elements, pitted and corroded. It is an object or the present invention to provide a new and improved process for providing a smooth and bright aluminum coating to ferrous metal objects.

Another object of the invention is to provide an improved process, which is both simple and inexpensive, for producing an aluminumor an aluminum base alloy coated ferrous metal product that is both bright and smooth.

A specific object of the invention is to provide an improved molten salt flux aluminizing process wherein the molten salt flux includes a chromium double salt utilized for purposes of fluxing the ferrous object and also for interacting with the molten bath of aluminum metal thereby to achieve a smooth, bright appearing aluminum coating on the ferrous metal object.

Further objects and features of the invention pertain to the compositions and arrangements utilized in achieving the above defined objects and will be better understood by reference to the following specification and single flow sheet drawing forming a part thereof.

In order for an aluminizing process to be economically feasible, the molten salt flux must contain inexpensive commercially available anhydrous salts that will, by combination or interaction with the steel base metal and the aluminum coating material, produce an adherent, smooth, corrosion resistant, bright-appearing aluminum coating. It has been found that by utilizing sodium chloride (NaCl) and potassium chloride (KCl) as the gross solvent, cryolite (Na AlF to act as a stabilizer and to provide a lower melting point for the molten salt flux and potassium chromium fluoride (K CrF or complementary chromium salts for effecting improved bonding, that the above identified objects can be achieved. The composition of the molten salt flux might be further modified by the inclusion of an iron oxide solvent such as aluminum fluoride (AlF or sodium fluoride (NaF) useful for cleansing the surface of the ferrous object.

In accordance with the best practice of the process in accordance with the invention, the ferrous metal object to be coated is first cleaned. The cleaning operation may include degreasing by use of an alkali cleaner or suitable solvents, pickling in a water solution of hydrochloric acid or sandblasting and then rinsing to obtain as clean a surface as possible on the ferrous object.

As a next step in the process, the cleaned ferrous metal object is immersed into a molten salt flux bath of the type described for purposes of fluxing the ferrous metal object, and thereafter the fluxed metal object is withdrawn from the molten salt flux bath and immediately. immersed into a molten bath of metal of the class including aluminum and aluminum base alloys. Specifically, t e molten b'athof metal-may be of aluminum alloyed with chromium, titanium and molybdenum metals, in appro- 2,963,384 Patented Dec. 6, 1960 2 priate proportions suited to give the desired finished product. In the final step of the process, the ferrous metal object is removed from the molten metal bath and the 'excess aluminum is removed as by draining, airblast or centrifuging so that the aluminum coating is solidified at the ambient temperature.

In the exercise of the process, it was discovered that a chromium double salt addition to the molten salt flux resulted in a brighter appearing aluminum coating on the ferrous metal object. Continued investigation disclosed that the chromium salt underwent decomposition when the fluxed ferrous article was removed from the molten salt flux bath and immersed into the molten metal bath of aluminum or aluminum base alloy. Spectographic analysis of aluminized coatings obtained by the process showed that the coatings contained, by weight, chromium in the range from a few hundredths of 1% to a few tltousandths of 1%. Confirming investigations utilizing small amounts of chrominum alloyed with the molten aluminum indicate that concentrations 'of chromium metal in the order of one-tenth of 1% by weight in the molten aluminum bath had a pronounced brightening effect on the appearance of the resulting aluminum coatings. While the physical mechanism that achieves this effect is not completely understood, it is apparent that the chromium double salt decomposes in the molten bath to permit the chromium to become alloyed with the aluminum. Accordingly, it is important to the practice of the invention that the ferrous object to be coated is moved directly and immediately from the molten salt flux bath to the molten bath of aluminum metal and in this sense it is advantageous to Percent by weight NaCl(sodium chloride) 35 to 55 -KCl (potassium chloride) 35 to 55 AlF (aluminum fluoride) or NaF (sodium fluoride) 0 to 12.5 Na AlF (cryolite) 5 to 20 K CrF (potassium chromium fluoride) 0.2 to 5 It is to be noted that the aluminum fluoride and sodium fluoride are iron oxide solvents which may be included in the bath but need not necessarily be included. As an example of a molten salt flux bath which is found to be effective in the practice of the process in accordance with the invention, the following formulation is offered:

Percent by weight NaCl 43 .5 KCl 43 .5 Na3AiF5 8-5 KgCI'FG 4.5

In the specific example, no aluminum fluoride or sodium fluoride addition was made because the ferrous articles used were thoroughly cleaned and there was lit-tie or no oxide surface film thereon. A small amount of aluminum metal was added to the molten flux bath for purposes of activation and as an aid in obtaining a defect free aluminized article. The molten salt flux was held at a temperature in the range of 1275 F. to 1400" F. and preferably at approximately l3tl0 P. and the clean ferrous articles were suspended in it for a period of from thirty seconds to two minutes. The ferrous objects so fluxe d were then removed from the flux bath and immediately dipped in a bath of molten metal, preferably commercially pure aluminum. The temperature of the molten metal bath was maintained and arranged froin 1275 to 1375 F. and the fluxed molten article was maintained therein for a period of from thirty seconds to two minutes, similar to the immersion period of the molten salt flux bath. Thereafter the articles were removed from the molten metal bath and drained of excess metal. The solidified aluminum coating so produced was smooth and bright in comparison to the rough, dull coated objects derived from processes employing molten flux baths not using a chromium double salt.

The ranges of temperature selected are considered optimum in that these temperatures will not result in excess fuming nor are they so low as to result in solidification of the bath if a great mass of metal is immersed therein. It is preferred that the salt used be as anhydrous as possible so that the water concentration limit for the entire bath is below 0.5 of 0.1% by weight.

Though the process has been described in terms of a molten metal bath made of commercially pure aluminum, it is appreciated that aluminum alloys such as that alloy identified as K726 made by Kaiser Aluminum Company, may be used in this arrangement. Without regard to the specific content of the molten aluminum bath, it is to be noted that activation of the molten salt flux can be achieved by placing the aluminum in contact therewith and that this activation can be achieved either by immersing a small amount of aluminum into the molten salt flux bath or floating the salt flux bath on top of the metal metal bath.

In view of the foregoing, it is obvious that there has been described herein an improved method and process for aluminizing ferrous metal objects and which improved process provides a smooth, bright appearing aluminum surface.

It is appreciated that variations and modifications may be made in the arrangement of the invention described herein and it is intended to cover in the appended claims all such variations and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A method of coating a ferrous metal object comprising the steps of preparing a fused salt bath composed by weight of:

maintaining said fused salt bath at a temperature in the range of 1275 F. to 1400 F., preparing a bath of molten metal of a class consisting of aluminum and of aluminum base alloys, immersing the ferrous metal object into said fused salt bath thereby to flux the surface of the object, transferring the fluxed ferrous metal object immediately from said fused salt bath into said molten metal bath thereby to coat the object with said molten metal, and removing the coated object from the molten metal bath to ambient temperature for solidifying the coating to the surface of said object.

2. The method of coating a ferrous metal object as set forth in claim 1 wherein said fused salt bath is activated by aluminum in contact with the fused salt.

3. The method of coating a ferrous metal object as set forth in claim 2 wherein said fused salt bath is activated by floating said fused salt bath on top of said molten metal.

4. A method of coating a ferrous metal object comprising the steps of preparing a fused salt bath composed by weight of:

maintaining said fused salt bath at a temperature in the range of 1275 F. to 1400 F., preparing a bath of molten metal of a class consisting of aluminum and of aluminum base alloys, cleaning the ferrous metal object to remove all foreign matter from the surface thereof, immersing the ferrous metal object into said fused salt bath thereby to flux the surface of the object, withdraw ing the fiuxed object from said fused salt bath and immediately immersing said object into said molten metal bath thereby to interact said molten metal with the fused salt on said object and to coat said fluxed object with said molten metal, and removing the coated object from the molten metal bath to ambient temperature for solidifying the coating to the surface of said object.

5. The method of coating a ferrous metal object as set forth in claim 4 wherein said cleaning step includes degreasing, pickling and rinsing the surface thereof.

6. A method of coating a ferrous metal object comprising the steps of preparing a fused salt bath composed by weight of:

range of 1275 F. to 1400 F., activating said fused salt bath by placing aluminum in contact with said salt, preparing a molten bath of aluminum metal alloyed with chromium, titanium and molybdenum metals, cleaning the ferrous metal object to remove all foreign matter from the surface thereof, immersing the ferrous metal object into said fused salt bath thereby to flux the surface of the object, withdrawing the fiuxed object from said fused salt bath and immediately immersing said object into said molten metal bath thereby to interact said molten metal with the fused salt on said object and to coat said fluxed object with said molten metal, and removing the coated object from the molten metal bath to ambient temperature for solidifying the coating to the surface of said object.

7. A method of coating a ferrous metal object comprising the steps of preparing a fused salt bath composed by weight of:

maintaining said fused salt bath at a temperature in the range of 1275 F. to 1400 F., activating said fused salt bath by placing aluminum in contact with said salt, preparing a molten bath of commerciallly pure aluminum metal, maintaining said molten metal bath at a temperature in the range of 1275 F. to 1400 F., cleaning the ferrous metal object to remove all foreign matter from the surface thereof, immersing the ferrous metal object into said fused salt bath thereby to flux the surface of the object, withdrawing the fluxed object from said fused salt bath and immediately immersing said object into said molten metal bath thereby to interact said molten metal with the fused salt on said object and to coat said fluxed object with said molten metal, and removing the coated object from the molten metal bath to ambient temperature for solidifying the coating to the surface of said object.

8. A method of coating a ferrous metal object comprising the steps of preparing a fused salt bath composed by weight of:

5 maintaining said fused salt bath at a temperature of substantially 1300" F., activating said fused salt bath by placing aluminum in contact with said salt, preparing a molten bath of commercially pure aluminum metal, maintaining said molten metal bath at a temperature in the range of 1275 F. to 1400 F., cleaning the ferrous metal object to remove all foreign matter from the surface thereof, immersing the ferrous metal object into said fused salt bath thereby to flux the surface of the object, withdrawing the fluxed object from said fused salt bath after immersion for thirty seconds and immediately im mersing said fluxed object into said molten metal bath for a corresponding period thereby to interact said molten metal with the fused salt on said object and to coat said fiuxed object with said molten metal, and withdrawing the coated object from the molten metal bath to ambient temperature for solidifying the coating to the surface of said object.

References Cited in the file of this patent UNITED STATES PATENTS Mauritz Dec. 7, 1937 2,544,671 Grange et al Mar. 13, 1951 

1. A METHOD OF COATING A FERROUS METAL OBJECT COMPRISING THE STEPS OF PREPARING A FUSED SALT BATH COMPOSED BY WEIGHT OF: PERCENT NACL ------------------------------------------ 35 TO 55 KCL --------------------------------------------- 35 TO 55 NA3ALF6 ----------------------------------------- 5 TO 20 K3CRF6 ------------------------------------------ 0.2 TO 5 MAINTAINING SAID FUSED SALT BATH AT A TEMPERATURE IN THE RANGE OF 1275*F. TO 1400*F., PREPARING A BATH OF MOLTEN METAL OF A CLASS CONSISTING OF ALUMINUM AND OF ALUMINUM BASE ALLOYS, IMMERSING THE FERROUS METAL OBJECT INTO SAID FUSED SALT BATH THEREBY TO FLUX THE SURFACE OF THE OBJECT, TRANSFERRING THE FLUXED FERROUS METAL OBJECT IMMEDIATELY FROM SAID FUSED SALT BATH INTO SAID MOLTEN METAL BATH THEREBY TO COAT THE OBJECT WITH SAID MOLTEN METAL, AND REMOVING THE COATED OBJECT FROM THE MOLTEN METAL BATH RO AMBIENT TEMPERATURE FOR SOLIDIFYING THE COATING TO THE SURFACE OF SAID OBJECT. 